Prostheses for the repair of vascular defects, including for example vascular aneurysms, are well known in the art. A common prosthesis for treatment of such a medical condition is a stent-graft.
Prostheses of this type are typically deployed endoluminally through a vein or artery adjacent a surface of a patient. For example, aortic prostheses are commonly fed through the femoral artery. A common method of deployment involves the location of a guide wire along the path to be followed by the introducer assembly, up to the site in the vasculature to be treated. Once the guide wire is in place, a series of catheters is advanced along the guide wire, finally with the introduction of an inner catheter assembly which carries the stent or stent-graft to be fitted. The catheters have sufficient trackability to follow the guide wire along the curves and turns of the patient's vasculature and some can also curve sufficiently so as to be able to fit a stent-graft, for example, into the aortic arch of a patient.
In order to guide the inner catheter assembly through the tortuous path of the patient's vasculature and along the guide wire, the assembly is typically provided with a flexible tip, most commonly in the form of a dilator tip although in some cases in the form of a flexible distal end of the inner catheter itself. The dilator or catheter tip is formed of a flexible material able to curve so as to follow the path of the guide wire over which is slides.
The implant is usually restrained on the inner catheter by one or more restraining mechanisms. Once the introducer has been located in the correct position in the patient's vasculature, with the implant positioned at the treatment site, the deployment procedure is commenced, typically first by the retraction of an outer sheath of the assembly to bare the implant. In the case of a self-expanding implant, the next step involves releasing the restraining system, which causes immediate or progressive expansion and thus deployment of the implant, in dependence upon its design and the nature of the restraining mechanism. In the case of an implant which is separately expandable, such as by balloon, the expansion mechanism is actuated after removal of the outer sheath to affect this expansion.
As a result of tension in various parts of the introducer assembly, particularly in the case of self-expanding implants and also in the case of deployment in a curved lumen such as in the aortic arch, as the implant is being released this tension in the assembly is typically released also. This can often result in a jerking action of one or more of the components of the system. Where the implant is to be located in a curved part of a patient's vasculature, this can result in the inner catheter being urged forwardly during the implant release phase. This can cause the distal end of the introducer to prod into the wall of the vessel, causing tissue damage or trauma. This can result in complications to the medical procedure, both during the course of the procedure itself and subsequently to that during the body's attempt to heal the vessel, in some cases resulting in the generation of stenosis. Even though the dilator tip is flexible, when this is prodded straight into a wall of a patient's vasculature, it is relatively hard and sufficiently so to cause damage to the vessel wall. The skilled person will realize that the reason for this increased hardness of the dilator tip in such circumstances is that such motion, instead of pushing the tip to effect sideways bending thereof, seeks to compress the tip longitudinally.